We would like to congratulate Drs Bernardini and Natalini [1] on their impressive anaesthetic database and their work in attempting to answer a very difficult question. However, we feel that there are several problems with their methodology, and thus their conclusions, with respect to the risk of aspiration, while using the laryngeal mask airway (LMA).
Firstly, we feel that the overall rate of LMA use in their population is low (38%), suggesting that the LMA was used only in low risk patients. Therefore, their results are difficult to extrapolate into clinical practice.
Secondly, the two study groups differed significantly in every measured characterestic of the database used. This is to be expected, as the anaesthetist choosing the airway device, will naturally use the airway device most suited to the patient. Some of the bias associated with these differences was addressed by propensity scoring. However, propensity scoring is not able to adjust for unknown variables. A number of the quoted factors increasing aspiration risk (e.g. hiatus hernia, history of reflux, opioid administration) are not in the list of database variables. Therefore, no adjustment would have been made for the bias introduced by the anaesthetist using these factors to assess risk, and assigning higher risk patients to the endotracheal tube group.
Thirdly, the authors state that 98.3% of patients with a contraindication to LMA received an endotracheal tube. However, we are not told how many patients, who were eligible for a LMA, actually received an endotracheal tube. Therefore, we do not know how well their criteria were applied, or whether any of the patients who aspirated (3 LMA, 7 ETT), should have been in the other airway group.
In conclusion, all we know now is that the two different groups of patients have the same risk of aspiration. The two groups will have different and unknown initial aspiration risks. Therefore, we do not know how the application of the different airway devices alters that risk.
Dr D.McPherson, Dr P.H.Dodd
Royal Hampshire County Hospital, Winchester.
Philip.Dodd@wehct.nhs.uk
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perioperative pulmonary aspiration in a university hospital: a retrospective 4 year analysis. Anesthesia &
Analgesia 2006; 103: 941-7.
3. Keller C, Brimacombe J, Bittersohl J, Lirk P, von Goedecke A. Aspiration and the laryngeal mask airway:
three cases and a review of the literature. British Journal of Anaesthesia 2004; 93: 579-82.
4. Warner MA,Warner WE, Webber JG. Clinical significance of pulmonary aspiration during the perioperative
period. Anesthesiology 1993; 78: 56-62.